Field of Invention:
This invention relates to xerography and more particularly to an improved apparatus for the development of an electrostatic image in which a toner layer is presented to a latent image for its development.
Description of Prior Art:
In the xerographic reproduction process, a photoconductive surface is charged and then exposed to a light pattern of the information to be reproduced, thereby forming an electrostatic latent image on the photoconductive surface. Toner particles, which may be finely divided, pigmentd, resinous material are presented to the latent image where they are attracted to the photoconductive surface. The toner image can be fixed and made permanent on the photoconductive surface or it can be transferred to another surface where it is fixed.
One known method of developing latent electrostatic images is by a process called transfer development. Transfer development broadly involves bringing a layer of toner to an imaged photoconductor where toner particles are transferred from the layer to the imaged areas. In one transfer development technique, the layer of toner particles is applied to a donor member which is capable of retaining the particles on its surface and then the donor member is brought into close proximity to the surface of the photoconductor. In the closely spaced position, particles of toner in the toner layer on the donor member are attracted to the photoconductor by the electrostatic charge on the photoconductor so that development takes place. In this technique the toner particles must traverse an air gap to reach the imaged regions of the photoconductor. In two other transfer techniques the toner-laden donor actually contacts the image photoreceptor and no air gap is involved. In one such technique the toner-laden donor is rolled in non-slip relationship into and out of contact with the electrostatic latent image to develop the image in the single rapid step. In another such technique, the toner-laden donor is skidded across the xerographic surface. Skidding the toner by as much as the width of the thinnest line will double the amount of toner available for development of a line which is perpendicular to the skid direction, and the amount of skidding can be increased to achieve greater density or greater area coverage.
It is to be noted, therefore, that the term "transfer development" is generic to development techniques where (1) the toner layer is out of contact with the imaged photoconductor and the toner particles must traverse an air gap to effect development (2) the toner layer is brought into rolling contact with the imaged photoconductor to effect development, and (3) the toner layer is brought into contact with the imaged photoconductor and skidded across the imaged surface to effect development. Transfer development has also come to be known as "touchdown development".
In a typical transfer development system, a cylindrical or endless donor member is rotated so that its surface can be presented to the moving surface of a photoconductive drum bearing an electrostatic latent image thereon. Positioned about the periphery of the donor member are a number of processing stations including a donor loading station, a which toner is retained on the donor member surface; an agglomerate removal station at which toner agglomerates are removed from the toner layer retained on the surface of the donor member; a charging station at which a uniform charge is placed on the particles of the toner retained on the donor surface; a clean-up station at which the toner layer is converted into one of uniform thickness and at which any toner agglomerate not removed by the agglomerate removal station are removed; a development station at which the toner particles are presented to the imaged photoconductor for image development; and a cleaning station at which a neutralizing charge is placed upon the residual toner particles and at which a cleaning member removes residual toner from the peripheral surface of the donor. In this manner, a more or less continuous development process is carried out.
Among the typical donor members employed in the process heretofore was a metal cylinder covered with an insulating enamel upon which was coated a metal electrode in a gravure-screen pattern. A potential of up to 300 volts is impressed between the electrode and cylinder while the cylinder is rotated in a vibrating tray of toner powder. In a mass of toner that appears to be electrically neutral there will be a roughly equal amounts of positively and negatively charged particles. Microsized electrostatic fields formed between the electrode and the cylinder cause toner of one polarity to deposit on the electrode and toner of the opposite polarity to deposit on the squares in the electrode. Clumps of excess toner are vacuumed off and the remaining uniformly thick toner layer is corona charged to make it all the same polarity, thus making the donor ready for use in developing an image.
As discussed previously the latent image on a photoconductive surface could be developed by mementarily "touching down" the donor member to the surface. The surface of the photoconductor containing the latent image is charged at a greater potential than the donor surface. Therefore, in charged areas of the surface, toner is attracted from the donor to the surface; in uncharged areas the toner-charge image forces keep the toner particles attracted to the donor, and the surface remains free of toner particles. However, it was found that several such "touchdowns" were needed to produce highly density images because of a sparse migration of toner particles from the donor to the photoconductive surface.
Thicker coatings of toner produced by various techniques were explored in attempts to obtain the density desired with one "touchdown", but these all seemed subject to the difficulty that where the thick coating of toner touched uncharged areas of the photoconductor surface, some surface toner particles less strongly attracted to the donor transferred to the photoconductor producing an objectionable background deposit. The obvious solution was to bring the donor only very close to the photoconductor but not into contact with it. Toner will jump across a narrow air gap to charged areas of a xerographic photoconductor surface, but not to uncharged areas. The images thus obtained were greatly improved. This latter process has been termed "spaced touchdown".
Developing across an air gap between the donor and the photoreceptor made it possible to produce background free images from heavily loaded donors. The gap was maintained by spacers at the ends of the rigid cylindrical donors and photoreceptors. However, it was soon determined that the gap spacing was critical dependent upon the toner loading characteristics of the donor.
With the typical donor member previously described, development can be carried out by rolling the donor cylinder in near contact with a charged and exposed xerographic photoconductive plate or drum. Spacing shims between the donor and photoreceptor, at the ends of the donor cylinder where there is not toner, maintain a space of about 0.001 to 0.002 inch between the surface of the toner and the surface of the photoconductor. During development a bias potential is applied to the photoconductor backing to compensate for any residual potential in background areas. If the bias potential is just equal in magnitude but opposite in polarity to the potential on the photoconductor in fully exposed areas, a good image will be produced, but some deposition of toner occurs in background areas. Such deposition of toner can be suppressed almost completely by increasing the bias potential to about 50 volts more than the background potential of the photoconductor. Thus, if the potential on the photoconductor in fully exposed areas is +100 volts, approximately -150 volts should be applied to the photoconductor backing. In the development step, spacing must be precisely controlled, and the voltage relationships between photoconductor and donor must be adjusted carefully to minimize background deposits without degrading fine line detail or lighter halftone tints.
The interrelationships between the various aspects of image quality and toner-layer thickness, toner charge levels, and spacing between donor and photoconductor can be summarized in domain plots of the type illustrated in FIG. 8, which applies for toner layers charged to potentials of about -200 volts. Generally acceptable images will be produced under the conditions indicated, for the central area between the two lines. As the donor-to-selenium (or photoconductor surface) spacing is reduced, background deposits will appear and become unacceptable. As the spacing is increased, image density drops and the ability of the process to reproduce fine lines and dots is reduced. Attempts to operate with toner layer thicknesses much less than 0.001 inch produce unsatisfactory images because the donor loading is generally not sufficiently uniform for such thin layers. In general, a donor to photoconductor surface spacing of between 0.001 to 0.010 inches, depending upon toner layer thickness can be used with acceptable results.
A microfield donor used in a "spaced touchdown" process can therefore produce good, high density images that do not have several of the defects commonly associated with images produced by toner-carrier developers. Although the processing steps are simple, there is a need to maintain accurate spacings to produce uniformly good images.